Integrated circuit manufacturing uses photolithography to transfer patterns from a master mask to a semiconductor substrate. As feature dimensions used in integrated circuits have become sub-micron various problems are encountered. The small dimensions approach the physical capability of photolithography. At such dimensions, the nonplanarity of surfaces, such as wafers have a significant affect on feature resolution on the wafer. A known compensation technique is the use of tiles which are additional features formed on the semiconductor substrate in areas that are not used for functional circuitry. Tiling is used to improve the planarity or flatness of the substrate surface after a conventional chemical/mechanical polish (CMP) step. Tiling can also be used to equalize the chemical concentration during an etch step and ensure that the concentration is uniform across a wafer. A major limitation with tiling is that tiling cannot be used in close proximity to functional circuitry because of negative effects on device characteristics.
Another known compensation technique to compensate light diffraction at submicron dimensions is referred to generally as sub-resolution features or scattering bars. This technique involves the placement of small features on the mask in close proximity to the small isolated desired design features. The scattering bars are placed within less than three times the minimum feature spacing. Scattering bars make isolated features pattern as if they are dense features in that they decrease the sensitivity to focus variations in a photolithographic system. Another known lithography problem is referred to as flare which is the existence of scattered background light in a lithographic system. Flare is dependent upon the pattern density of a mask. As flare varies, there is also variation in wafer feature dimensions. Scattering bars may offer some improvement in highly local flare reduction and pattern uniformity. However the scattering bars do not provide improvement for medium or long range pattern density or flare distortion.
Another known lithographic issue is the variation of mask or reticle feature dimensions as mask pattern density varies. Tiling offers some improvement to this issue but tiling is limited in use due to the electrical modifications on functional devices as mentioned above. Subresolution scattering bars do not offer significant improvement to this problem. For both flare and reticle feature dimensions, there are solutions known as biasing feature edges. These solutions are generally referred to as optical proximity correction (OPC). A problem with these solutions is that they require extremely intense computation for every feature edge as to what is the impact of flare or reticle pattern density.
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